1. Field of the Invention
The invention relates to apparatus and methods for light spot servo signal detection, and more particularly to apparatus and methods for track center error and tilt error detection of optical discs.
2. Description of the Related Art
FIG. 1A shows a conventional optical storage device. Conventionally, data stored in an optical disc is amplified and digitized to a target level before decoding. The variable gain amplifier 102, analog to digital converter 104 and auto gain controller 106 form an AGC loop to adjust the gain of the RF signal #RF. An extra data path is formed by a blank detection unit 110 to detect blankness of the RF signal #RF, where the blankness is corresponding to at least a blank sector of a track on the optical disc. If amplitude of the RF signal #RF is below a predetermined threshold, the decoder 108 is not enabled to decode data, and the corresponding sector is reported as blank. Otherwise, if the RF signal #RF is not blank, the blank detection unit 110 sends an enable signal #en to the decoder 108, enabling the decoder 108 to decode the data signal #DATA output from analog to digital converter 104.
FIG. 1B is a schematic view showing the situation of blankness. When the amplitude of the RF signal #RF is below the threshold (+th and −th), the corresponding sector where the RF signal #RF is obtained is reported as a blank sector. When the amplitude of RF signal #RF exceeds the threshold, the decoder is enabled to decode the data signal #DATA.
FIG. 2 shows a conventional loop control circuit for implementation to an electrical device, such as an optical storage device. The loop control circuit typically comprises an auto gain control loop formed by variable gain amplifier 202, analog to digital converter 204, peak bottom detector 206 and auto gain controller 208, and an offset control loop formed by variable gain amplifier 202, analog to digital converter 204, offset controller 210 and adder 212. The variable gain amplifier 202 amplifies an RF signal #RF received from a front end, such as an optical disc (not shown) before transmission to the analog to digital converter 204. The RF signal includes data information therein. If the amplitude of RF signal #RF is not within a proper range, the analog to digital converter 204 may not correctly sample the RF signal #RF to generate the digital data signal #DATA. Thus, the auto gain controller 208 generates a gain value #gain to control the amplification of RF signal #RF, and the gain value #gain is determined by detection results of the peak bottom detector 206. The auto gain controller 208 utilizes a step size to update the gain value #gain according to peak and bottom levels #PB sent from the peak bottom detector 206, and the gain control loop is recursively processed to gradually approximate the amplitude of data signal #DATA to a target level. Therefore the step size may also be referred to as a loop convergence ratio. Likewise, the offset controller 210 detects offset of the data signal #DATA and generates an offset signal #offset to compensate RF signal #RF. The RF signal #RF may be directly added by the offset signal #offset in the adder 212 before transmission to the variable gain amplifier 202, and the offset signal #offset is recursively and gradually updated by another step size provided in the offset controller 210. In this way, the offset of RF signal #RF is gradually corrected through the feedback mechanism.
FIG. 3 shows waveforms of various conditions. The RF signal #RF obtained from the front end may not be at a proper level for post processing. For example, in period t1, the amplitude of RF signal #RF is below the target level (+target and −target), and the gain control loop gradually amplifies the RF signal #RF to approximate the target level. In period t2, the amplitude of RF signal #RF exceeds the target level, and the gain control loop works to reduce it. Periods t3 and t4 show examples of offset compensation. The offset control loop as described in FIG. 2 gradually adjusts the RF signal #RF through feedback control, thus the RF signal #RF is maintained within the target level before transmission to the analog to digital converter 204. Time required for the control loops to approximate the RF signal #RF to the target level, however, may be inefficient. If the convergence ratio of the control loops is set too low, a long period of time is required before the RF signal #RF reaches the target level. Otherwise, if the convergence ratio is set too high, the control loops may be unstable, reducing signal quality for the analog to digital converter 204. Thus, determination for step sizes of the gain control loop and offset control loop is an important issue.
Please refer to FIG. 4. FIG. 4 is a cross-sectional diagram illustrating an optical medium. As shown in FIG. 4, the innermost area is the inner drive area, then the lead-in zone, the data zone, lead-out zone, and the outer drive area. The inner drive area includes different sub-zones such as the initial zone, the inner disc test zone, the count zone run-in, the inner disc count zone, the inner disc administration zone, and the table of contents zone. The inner disc test zone is disposed for the optical storage drive to perform disc tests and Optimized Power Control (OPC) algorithms. The optical storage drive emits laser beams with various power levels onto the inner disc test zone of an optical storage medium to form a plurality of marks. Then, the reproduction signals from those marks are captured as reference information for adjusting emitted power level. Thus, the optical storage drive can optimize the power level of the emitted laser beams.
The optimized power is determined according to the asymmetry of the waveform of the recorded data reproduction signals. In the prior art, the asymmetry of the waveform of the recorded data reproduction signal is measured in analog domain, costing much layout space and raising the design complexity.
Signal quality can deteriorate significantly with servo error such as tilt and mis-track of a disc as recording density becomes higher not only in a disc only for reproduction such as a DVD-ROM but also in a recordable disc such as a DVD-RAM. In particular, in the recordable disc, the recording quality deteriorates due to the influence of the servo error when the servo error occurs during recording and the deterioration of the quality of the signal becomes severe due to the servo error during the reproduction of an applicable part.
In a DVD-RAM disc, information is recorded on a track comprising a land track and a groove track. The land track and the groove track alternate when the disc rotates one circle (360 degrees). The land track and the groove track are alternated in the DVD-RAM disc to provide a tracking guide in an initial stage and reduce crosstalk between adjacent tracks in high density narrow tracks.
Each track comprises sectors having a uniform length. A pre-embossed header area is provided during the manufacturing of the disc as a means of physically dividing the sectors. The physical addresses of the sectors are recorded in the pre-embossed header area. Each sector comprises a data area and a header area in which physical identification data (PID) is recorded.
FIG. 5A shows the physical shape of the land track in a DVD-RAM disc. FIG. 5B shows the waveform of a Read channel 1 signal in the land track. The header area is repeatedly arranged in every sector of the track. Four PIDs (PID1 through PID4) having the same value are recorded in one header area. The PID1 and the PID2 are arranged to deviate from the center of the track by a certain amount and the PID3 and the PID4 are arranged to deviate from the center of the track in a direction opposite to that of the PID1 and PID2 so that the PIDs can be correctly read even if a laser spot 500 deviates from the center of the track. The Read channel 1 signal shown in FIG. 5B can be obtained in the land track, wherein ISHD1, ISHD2, ISHD3, and ISHD4 are respectively DC bottom values of variable-frequency oscillator (VFO) signals of fields Header1, Header 2, Header3, and Header4. Also, the arrangements of the PID1 and PID2 and the PID3 and PID4 in the land track are opposite to those in the groove track. FIG. 6A shows the physical shape of the groove track in a DVD-RAM disc. FIG. 6B shows the waveform of the Read channel 1 signal in the groove track.
FIG. 7 shows the enlarged header area shown in FIGS. 5A and 6A. In the structure of the header area, the PID1 and PID2, and the PID3 and PID4 are arranged to deviate from the center of the track in opposite directions by a uniform amount. The VFO signal having a specified frequency for synchronizing and detecting ID and an ID signal showing the physical addresses of the sectors are recorded in the respective PIDs. The VFO signal has a recording pattern of 4 T (T is a period of the clock signal). As shown in FIG. 7, the header area comprises VFO1 area 701 and PID1 702, VFO2 area 703 and PID2 704, VFO3 area 705 and PID3 706, and VFO4 area 707 and PID4 708. In FIG. 7, when the laser spot 700 passes through the header area of the groove track, a Read channel 1 signal #RF shown in FIG. 8 is obtained. In FIG. 8, a VFO1 signal 802 corresponds to VFO1 area 701 of FIG. 7. A VFO3 signal 803 corresponds to VFO3 area 705.
FIG. 9 shows a conventional apparatus detecting track center error and tilt error of a DVD-RAM disc. Peak detection circuit 901 detects peak values of Read channel 1 signal #RF and generates peak signals, and bottom detection circuit 902 detects bottom values of Read channel 1 signal #RF and generates bottom signals. Sample hold circuits 903A and 903B respectively sample the peak and bottom signals in areas VFO1 and VFO3, and hold the sampled signal until as being sampled by analog to digital converters (ADC) 905A and 905B which are low sampling rate ADCs. Track center error detector 907 calculates track center error with peak and bottom values sampled by ADCs 905A and 905B. Tilt error detector 909 calculates tilt error with bottom values sampled by ADC 905B.
However, sample hold circuits 903A and 903B are analog circuitries, which have poorer accuracy opposite to digital circuitry. In addition, the sample times of sample hold circuits 903A and 903B are limited by their switching frequency. Thus, it is difficult to detect Read channel 1 signal #RF more frequently in a short period of time using the analog sample hold circuits 903A and 903B, deteriorating the detection accuracy. To detect Read channel 1 signal #RF more frequently in a short period of time, more complexity sample hold circuits are required, however, increasing the cost and size of the circuit detecting track center error and tilt error of a DVD-RAM disc.